Ipyflow

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* [kernel] handle binary ops in liveness checker;

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* [kernel] improve precision of timestamp updates for mutation handlers;

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* [kernel] turn off tracing for InteractiveShell API;
* [kernel] improve edge precision around nested symbols;

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* [kernel][lab] improve accuracy of readiness;
* [kernel][lab] improve accuracy of pull-based reactivity;

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* [kernel] integration with mercury widgets;
* [kernel] improve accuracy of static write detection;

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* [lab] Reactivity by default (!!!)
* [lab] Improvements to reactive execution engine: update stale / dirty dependencies as well;
* [lab] Initialization improvements after kernel is started / restarted;
* [lab] Use a more a11y-friendly color scheme by default;
* [lab] Support any-order execution semantics in batch reactive mode;
* [kernel] Add a `reproduce_cell` API helper function;
* [kernel] Better static edge detection / removal;
* [kernel] Improvements to `%%memoize` magic (--verbose and --quiet specifiers);
* [kernel] Improvements to resync logic if tracing disabled;
* [kernel] Fix multithreading bug;
* [kernel] Support `NoneType` as a subscript type;
* [kernel] Misc stability improvements and bugfixes;